The present invention relates to a method for controlling an exposure amount when printing an original image of a photographic film on a photographic paper. The invention relates more particularly to an exposure control method for photographic printing wherein an original image on a photographic film is scanned, consequent image information obtained from the scanning is analyzed, and an exposure amount based on an average photometric value of transmitted light is properly corrected by the results of the analyses so that satisfactory photographic prints may be made stably and easily.
In ordinary photographing, it is a known empirical principle that the average reflectance for each of the three primary colors, blue (B), green (G) and red (R) of a subject (hereinafter referred to simply as B, G and R) is almost constant. In a conventional photographic printing apparatus, therefore, large area transmission density (LATD) of a photographic original image has been measured, and an exposure amount for photographic printing has been determined based on the measured LATD (hereinafter referred to as an LATD method). When a film on which a standard subject is photographed is used for printing, it is possible to obtain photographic prints having excellent color balance by keeping an exposure amount for each of B, G and R light-sensitive layers of a photographic paper constant by means of the above-mentioned exposure control method.
However, when a subject has its own bias in its brightness distribution or its color distribution (which is called a subject failure), it is difficult to obtain satisfactory photographic prints through the above-mentioned LATD method. A subject failure wherein a bias of brightness distribution of a subject is remarkable is called a density failure and a subject failure wherein a bias of color distribution is remarkable is called a color failure.
As known technology to correct an exposure amount for the density failure, Japanese Patent Examined Publication No. 2691/1981 may be cited. In this technology, an original image on a photographic film is scanned, and from the image density obtained from the scanning, "specific values for location and density" are obtained for each region on the image. Then, classification in terms of the specific values is made, and a function of predetermined specific values may adjust, together with the aforesaid classification, the exposure amount for the original image.
On the other hand, as known technology to correct an exposure amount for the color failure, "Exposure Determination Methods for Color Printing: The concept of Optimum Correction Level" written by C.J. Bartleson and R.W. Huboi and J. of SMPTE, 65, 205-215 (1956), for example, may be cited.
This paper contains the following descriptions.
1 Full correction in which the exposure is adjusted depending on LATD and thereby an exposure amount for the light-sensitive layer of each color is made constant, is effective for photographic originals of photographed standard subjects.
2 No correction in which an exposure is conducted under the condition of a constant light flux or a constant exposure time is effective, for a subject failure in the case of optimum exposure for photographing.
Further, the following point is described in the paper as an actual compromise.
3 A lowered correction that is a control method positioned between the above-mentioned 1 and 2 is suitable, and the optimum correction level should be selected from overall print quality for the entire population.
In a recent photographic printing apparatus, in general, correction levels of several steps can be selected, and an operator who observes photographic originals can select the correction level for each original so that he may adjust a color balance. Furthermore, for the selection of the correction level, when lighting for photographing is improper or the original is affected by latent-image decay, the full correction is suitable, and when a subject is biased in terms of colors, the lowered correction is usually considered optimum.
In "Modern Exposure Determination for Customizing Photofinishing Printer Response", JAPE, 5, 93-104 (1979), there is suggested an exposure determination method for adjusting the correction level by paying attention to hue, because the influence of color temperature of lighting for photographing is remarkable for the specific hue in chromaticity computed by the use of an average density of a photographic original. However, there is no big difference of chromaticity obtained from the average density, between, for example, a photographic original made through photographing under a fluorescent light and a photographic original of a green lawn. Therefore, it is difficult to discriminate between them based on their hues.
As mentioned above, the selection of a correction level by means of an average density is not always effective. In a photographic printing apparatus of an LATD control system, it is also impossible to discriminate automatically whether density variation of each color of B, G and R of a photographic original is caused by a tone reproduction characteristic of a photographic film or by a bias of color distribution of a subject. In photographic printing apparatus of this type, therefore, appropriate conditions for exposure are established in advance for each type of photographic film, and exposure conditions are selected through switching based on the type of photographic film used for printing.
Recently, however, the number of types of photographic films has been greatly increased. Therefore, establishment of exposure conditions for each type of film and the switching operation for selecting the exposure conditions mentioned above are major factors impeding enhancement of efficiency in the photographic printing process. Further, even in the same type of photographic film, exposure conditions established in advance sometimes do not provide photographic prints having a proper color balance for photographic originals whose characteristics have been deteriorated because of improper storage conditions of the film after photographing. Existence of such film is also a serious obstacle for stable and efficient production of photographic prints having constant quality.
For the problems mentioned above, Japanese Patent Publication Open to Public Inspection No. 46741/1980 (hereinafter referred to as Japanese Patent O.P.I. Publication) discloses a method wherein a photographic film is color-separated into three primary colors to be scanned, neutral density and two sets of density differences between two colors are obtained based on image density of each color obtained through the scanning, a value specific to the photographic film is obtained from the relation of functions of the neutral density and two sets of density differences mentioned above, thus the specific value is used for exposure control.
In addition to the above, Japanese Patent O.P.I. Publication No. 6939/1990 discloses a method wherein a plurality of photographic originals are color-separated to be scanned, a cumulative distribution function of the image density obtained through the scanning for each color is obtained, and an exposure amount is determined based on the function mentioned above.
In the aforementioned exposure determination methods, however, it is not possible to discriminate whether the density variation for each of B, G and R colors of a photographic original is caused by photographing lighting or by color distribution on a subject both differing for each photographic original, though it is possible to discriminate whether the density variation for each of B, G and R colors of a photographic original is caused by a tone reproduction characteristic of a photographic film or by color distribution on a subject. Therefore, the above-mentioned exposure determination methods can not be free from the problem that it is not possible to obtain photographic prints having excellent quality from photographic originals obtained through photographing under improper lighting.
Furthermore, in the examples mentioned above, an exposure amount is determined based on image density information obtained through scanning by means of an image sensor. However, the sensitometric dynamic range for ordinary image sensors represented by a CCD is narrow for the extremely broad density range of a photographic film, which is a disadvantage. Therefore, it is difficult to determine, with high reproducibility, an exposure amount for a photographic original by scanning accurately and stably the density of the photographic original with the image sensors mentioned above.
In order to broaden a sensitometric dynamic range, on the other hand, it is necessary to arrange, in the form of a line or a plane, photoelectric conversion elements each having a large light-receiving area, which is extremely expensive. There is further a problem that the image sensor having a large external dimension can not be free from a large scale image pickup system including an image forming optical system and a color separation means. In addition to that, signal processing for output from a plurality of image sensors at a high speed and at a high S/N ratio requires a complicated and expensive circuit construction, which is a disadvantage.
In the methods mentioned above, it is necessary to determine an exposure amount by scanning, in advance, a photographic original prior to the exposure. Therefore, there is required an apparatus construction in which a scanning section and an exposure section are separated. However, such separated construction causes great inconvenience in the case of reprinting and print remaking. That is, reprinting or print remaking is on the assumption that photographic prints made therein should be the same as those obtained in the initial printing. Therefore, the image pickup system used for the initial printing should be used again for scanning of the photographic original in reprinting or print remaking. However, it is not easy to scan a short-cut film continuously for exposure, and both a scanning unit and an exposure unit require a light source and a film conveyance system. In addition to that, the control of conveyance of photographic films can not be free from complication. Therefore, an apparatus that materializes the aforesaid method tends to be extremely expensive, which is a problem.
Further, illumination light to a photographic original in an exposure system varies due to an aging change of a light source and an optical system, or a dye image of the photographic original is deteriorated by radiant heat generated during illumination for exposure. In that case, it is substantially impossible to detect the variation and adjust an exposure amount in equipment wherein a scanning unit and an exposure unit are separated in terms of space and time. Consequently, it is not possible to avoid a difference between the finish of prints in the initial printing and that of prints in reprinting or print remaking, which is a disadvantage.